In the fabrication of modem semiconductor devices, the deposition of a thin layer on a semiconductor substrate by chemical reaction of gases is one of the primary steps. Such deposition process is referred to generally as chemical-vapor deposition (CVD). Typical thermal CVD processes supply reactive gases to the substrate surface where heat-induced chemical reactions take place to produce a desired layer.
Plasma-enhanced CVD (PECVD) techniques, on the other hand, promote excitation and/or dissociation of the reactant gases by the application of radio frequency (“RF”) energy to a reaction zone near the substrate surface, thereby creating plasma. The high reactivity of the species in the plasma reduces the energy required for a chemical reaction to take place, and thus lowers the temperature required for such CVD processes as compared to conventional thermal CVD processes. These advantages are further exploited by high density plasma (HDP) CVD techniques, in which dense plasma is formed at low vacuum pressures so that the plasma species are even more reactive.